It is well known in the commercial production of crude acrylic acid by the oxidation of acrolein as well as in the commercial production of methacrylic acid by the oxidation of methacrolein that various byproducts are invariably also formed, including acetic acid. More particularly and by way of example, in the production of acrylic acid these various byproducts can include, in addition to acetic acid, propionic acid, water, acrylic acid dimers, acrolein, benzaldehyde, furfurals and hydroquinone. Conventionally, manufacturers have been led to adopt a number of purification methods to remove these byproducts to an extent whereby the purified acrylic acid (glacial acrylic acid) can be used in subsequent conversions, especially in polymerization processes wherein the purified acrylic acid or an acrylate prepared therefrom is used as a monomer.
Acetic acid and propionic acid are recognized as particularly problematic in that both are saturated and cannot be polymerized, so that depending on the polymerization process involved and the applications targeted for the polymer, these impurities may remain in the finished product and risk conferring undesirable corrosive properties on the finished product or being reencountered as waste in the liquid or gaseous discharges from the polymerization process. While propionic acid is removed from the crude acrylic acid product only with great difficulty and at significant expense, inasmuch as the difference in boiling points between propionic and acetic acids is very small, acetic acid can be removed to an extent in a light fraction by conventional distillation methods.
Unfortunately, while the amount of byproduct acetic acid produced varies in known processes for making acrylic acid or methacrylic acid through acrolein and methacrolein intermediates, respectively, dependent in part on what starting material or materials are employed, whether propylene or glycerol for acrylic acid manufacture or isobutene, isobutyraldehyde, t-butanol, isobutanol and the like for methacrylic acid manufacture—for example, up to about 5 percent by weight for acrolein produced from propylene and up to about 10 percent by weight for acrolein produced from glycerol—nevertheless the amounts produced are all substantial as compared to a desired content of less than about 0.1 percent by weight of acetic acid in a glacial acrylic acid finished product. Consequently, whatever the feed and process for making acrylic acid via acrolein oxidation and/or for making methacrylic acid via methacrolein oxidation and whatever the particular method used or proposed to be used for separating out the substantial byproduct acetic acid, a substantial yield loss of acrylic acid has accompanied or has been understood as necessarily coincident with removing the acetic acid down to the requirements of glacial acrylic acid, and this is particularly so with the production of a biobased acrylic acid as intensively researched in recent years.
Acrylic acid has also been prepared by oxidative methods other than through an acrolein intermediate. More particularly, methods have been described for making acrylic acid from propane according to a redox mechanism. U.S. Pat. No. 6,833,474 to Dubois, U.S. Pat. No. 7,332,625 to Dubois et al. and U.S. Pat. No. 7,345,198 to Dubois et al. provide examples of such methods from one company, wherein a gaseous mixture comprising propane, steam or water vapor and optionally an inert gas is passed over catalysts of a prescribed character in the further presence of molecular oxygen (U.S. Pat. No. 7,332,625 and U.S. Pat. No. 7,345,198) or in the absence of molecular oxygen (U.S. Pat. No. 6,833,474), and the propane is oxidized to acrylic acid. In these oxidative methods, as well, acetic acid is produced as a byproduct.